The present invention relates to a system for two-way communication between a master station and a plurality of substations via a satellite acting as a transponder wherein the transmission of information from the substation to the master station takes place in a time division multiple access (TDMA) mode, with each substation transmitting its information to the satellite in the form of periodic bursts on a radio-frequency channel common to all substations, and fixing the instants of its burst transmissions with respect to the instant of reception of a reference frame sync signal transmitted from the master station to all substations via the satellite.
A communication system of this kind is known, for example, from U.S. Pat. No. 3,742,498. In that system, the TDMA frame contains the bursts from the substations and from the master station and the reference frame sync signal or reference burst additionally transmitted by the master station. Each substation receives from the master station the reference frame sync signal and the data transmitted within that time slot of the TDMA frame assigned to the main station as far as this data is addressed to one of the substations. The system is suited to two-way communication between a ground control center and a plurality of aircraft via a satellite, but requires expensive synchronization equipment on the ground and aboard the aircraft because the constantly changing distances between the satellite and the aircraft have to be taken into account during synchronization to prevent time overlapping of the burst of the TDMA frame at the satellite. Correspondingly expensive equipment is required in this system to permit non-interfering first access by an aircraft.
If the positions of the aircraft are to be determined using conventional navigation equipment instead of such an aircraft-satellite-ground control center communication system, and if the amount of information transmitted is not so large that only short guard times between the different bursts on the order of one microsecond (.mu.s) are possible, a simpler communication system will be sufficient to handle the data to be exchanged between a ground control center and a plurality of aircraft for the purpose of air-traffic control.